The ups and downs of linear resonant acceleration

Most people familiar with linear resonant accelerators have the goal of driving it accurately, but due to variables outside their control, that goal isn't always accomplished

Auto resonance tracking is a feature that allows you to detect the current resonant frequency of a linear resonant actuator in your system. You can find it on haptic actuators like TI’s DRV2603 (as well as future haptic drivers). By isolating and finding the accurate resonant frequency, you can maximize the value obtained from your actuator and haptic-enabled system. LRAs become driven more efficiently and consume less power, enhancing your end products’ marketability. This calibration is an ongoing function being checked every time a haptic waveform is initiated, not just once at device startup.

This is because the system’s spring constant can change at a given moment’s notice. For instance, say I’m holding my phone with two hands. I’m using one hand to hold the phone and the other to make a menu selection, moving about all the while. But later in the day, I’m playing a game on my phone that is now resting on a table. These are two completely different use cases. Therefore, the system needs to be driven at two different resonant frequencies.

Using auto-resonance tracking the haptic driver on board can sense the change in the actuator’s performance style and adjust it up or down to suit the delta. This ensures power savings with a performance boost guaranteed upon every operation of your haptic system – not only at boot up. The DRV2603 even takes it a step farther to adjust the frequency of your input signal to ensure it’s in the device’s correct band of operation. You don’t even have to worry about getting it completely right. It takes care of that for you, so you can worry about more important things, like gaming options!

Auto resonance tracking simplifies your design in addition to all those benefits previously mentioned. Since it is more or less auto-tuning, you only need one external component for the entire solution size (and that is just a capacitor for power). So in addition to saving your power and upping the output strength, it also saves you money by reducing your overall board space needs.

Conclusion

Haptics has been slowly making its way into the accepted majority of mainstream society. It seems that with each iteration of mobile electronics an improvement is made to some aspect. Here we covered why linear resonant actuators are a great example of that; how existing motor-based technology has been revolutionized from previous mature implementations; boosting vibration strength while lowering power consumption. The only caveat is fulfilling that goal of maintaining the resonant frequency across all conditions. With innovative techniques such as auto-resonance tracking in newer haptic drivers that goal has been fulfilled, leaving you with time to focus on the more important aspects of your design ensuring that you meet your goal.

Eric Siegel is the Touch Business Development Manager for touch screen controllers and haptics drivers at Texas Instruments. Eric also worked as a test engineer for digital signal processors and in marketing for microcontroller solutions. He has an MSEE from the University of Florida. Eric can be reached at ti_ericsiegel@list.ti.com.

Good intro article. I have tried your DRV2603 LRA driver and it works very well. I would be interested in a follow up article that goes into detail about how auto-tune resonant frequency detection works.

It did take a bit to understand what the actual subject was, it is quite interesting. I can visualize at least one way to track the actual resonant frequency, I am sure that there are a few more. The simplest way is to use the device in the feedback loop of it's driving oscillator, like the tuning fork oscillators used in the past. That would be the cheap and easy approach and it would not need a microcontroller or any software to make it work. So probably nobody would want to do it that way.

It looks like everybody else calls these linear resonant actuators, not accelerators. Once I figured out the terminology difference, it was much easier to find more information about their applications.